The stages of development of mammals are. Embryonic development of mammals

An embryo (synonymous with embryo) is an organism that develops inside the egg membranes or in the mother's body. Under the embryonic, or embryonic, development in humans is understood the early period of development of the body (up to 8 weeks), during which a body is formed from a fertilized egg that has the main morphological features of a person. After 8 weeks, the developing human body is called a fetus (see).

Embryonic development is divided into a number of periods.
1. The period of a unicellular embryo, or zygote, is short-term, flowing from the moment of fertilization to the start of egg crushing.

2. Crushing period. During this period, cells occur. The cells obtained during crushing are called blastomeres. First, a bunch of blastomeres is formed, resembling a raspberry in shape - a morula, then a spherical single-layer blastula; the wall of the blastula is the blastoderm, the cavity is the blastocele.

3. Gastrulation. A single-layer embryo turns into a two-layer one - gastrula, consisting of an outer germ layer - ectoderm and an inner one - endoderm. In vertebrates, already during gastrulation, a third germ layer, the mesoderm, also appears. In the course of evolution in chordates, the process of gastrulation became more complicated by the appearance of an axial complex of rudiments (anlage of the nervous system, axial and musculature) on the dorsal side of the embryo.

4. The period of isolation of the main rudiments of organs and tissues and their further development. Simultaneously with these processes, the unification of parts into a single developing whole is intensifying. Skin is formed from the ectoderm nervous system and partially, from the endoderm - the epithelium of the alimentary canal and its glands; from mesoderm - muscles, epithelium genitourinary system and serous membranes, from the mesenchyme - connective, cartilaginous and bone tissues, vascular system and blood.

When conditions change, the course of development of individual parts of the embryo may change, and the germ layers may give rise to organs and tissues that are not those that should have developed from them under normal conditions. Factors that change the conditions of development can be environment(its chemistry, temperature, etc.), the interaction between the parts (cells, rudiments) of the embryo itself, as well as heredity. All these factors are closely related.

Rice. 1. Scheme of the early stages of development of the human embryo: a - stage of the internal cell mass; b - an eight-day embryo; c - twelve-day embryo; g - thirteen, fourteen-day embryo. 1 - trophoblast; 2 - blastocele; 3 - amnion cavity; 4 - endoderm cells; 5 - amnion; 6 - embryo; 7 - yolk sac; 8 - mesoderm cells; 9 - stalk; 10 - villus of the chorion; 11 - extra-embryonic whole.

Rice. 2. The embryo and its membranes at the early stages of development (a - c - successive stages): 1 - chorion; 2 - allantois; 3 - yolk sac; 4- amnion; 5 - extra-embryonic whole; 6 - umbilical cord; 7 - umbilical vessels; 8 - amnion cavity.

Rice. 3. Human embryo: a - by the end of the 4th week; b - by the end of the 5th week; c - by the end of the 7th week after fertilization.

In humans, fertilization occurs in the (oviduct). The crushing process takes place within 3-4 days, when the embryo moves through the fallopian tube to the uterus. As a result of crushing, a shell is formed from the surface blastomeres, which is involved in the nutrition of the embryo - the trophoblast. The central blastomeres form the embryoblast, from which the body of the embryo develops. Within 4-6 days the embryo is in the uterine cavity. With the beginning of the second week, the embryo sinks into the wall of the uterus (implantation). In a 7.5-day embryo, it forms an amniotic vesicle, the part of which, facing the endoderm, is the ectoderm of the embryo. During this period, the embryo has the shape of a shield (disk). Cells of the extraembryonic mesenchyme move out of it into the cavity of the blastocyst and fill it (Fig. 1). Together with the trophoblast, it forms the villous membrane of the embryo - the chorion (see). By the end of the second week, it forms a yolk sac. As a result of the fouling of the amniotic and yolk sacs with mesenchyme, the amnion and the yolk sac are formed.

"Extra-embryonic" parts play an important role in the development of the embryo. The yolk sac in the human embryo functions only in the early stages of development, participating in the nutrition of the embryo and performing a hematopoietic function. Allantois in oviparous higher vertebrates acts as a urinary sac, in humans it is a finger-like outgrowth of the hindgut, along which it grows to the chorion. Amnion - a water shell - forms a closed sac around the embryo, filled with liquid - amniotic fluid. It protects the embryo from harmful influences and creates favorable conditions for its development (Fig. 2).

At the 3rd week of development, a dense strand of growing cells stands out on the dorsal side of the embryo - the primary strip, the head section of which thickens and forms a primary (Hensen's) nodule. Cells of the primary streak plunge into the primary groove, penetrate into the space between the ectoderm and endoderm, and give rise to the middle germ layer. On the 3rd week, the dorsal string and neural tube are laid.

At the 4th week, the embryo separates from the extraembryonic parts and, as a result of increased growth, rolls up into a tube. At the same time, the mesoderm differentiates and body segments, somites, are formed (Fig. 3a). In parallel with segmentation, the initial processes of organogenesis (see) and histogenesis are performed. On the 5th week, the rudiments of the hands appear, and then the legs, on the 6th they are divided into the main sections, on the 7th the rudiments of the fingers appear (Fig. 3, b and 3, c). At the age of 8 weeks, the embryo acquires the main morphological features of a person in appearance and in internal organization. Its length (from the crown of the head to the coccyx) is 4 cm, weight is 4-5 g. By the end of the 8th week, the laying of the organs of the embryo ends.

The process of human embryonic development has 4 stages, and in time it lasts 8 weeks. It begins from the moment of the meeting of the male and female germ cells, their fusion and the formation of a zygote, and ends with the formation of an embryo.

After the fusion of the sperm with the egg, education It is she who moves through the fallopian tubes for 3-4 days and reaches the uterine cavity. At the same time, a period is observed. It is characterized by strong intensive cell division. At the end of this stage of embryonic development blastula is formed- accumulation of individual blastomeres, in the form of a ball.

The third period, gastrulation, involves the formation of a second germ layer, resulting in gastrula is formed. After this, a third germ layer appears - the mesoderm. Unlike vertebrates, embryogenesis in humans is complicated by the development of the axial complex of organs - the rudiments of the nervous system are laid down, as well as the axial skeleton and, along with it, the muscles.

During the fourth stage of development of the human embryo, segregation of educated present moment rudiments of future organs and systems. Thus, the aforementioned nervous system is formed from the first germinal layer, and partly the sense organs. From the second endoderm - the epithelial tissue lining the alimentary canal and the glands located in it. From the mesenchyme, a connective, cartilaginous, bone as well as the vascular system.

The stages of human embryonic development, presented in the table below, do not always go in the order in which it is necessary. So, under the influence of a certain kind of factors, mainly exogenous, the course of development of individual organs and systems can be disturbed. Among these reasons can be identified.

The subtype of mammals is very diverse in terms of the nature of embryogenesis. The complication of the structure of mammals, and, consequently, of embryogenesis necessitates the accumulation more nutrients in eggs. At a certain stage of development, this supply of nutrients cannot satisfy the needs of a qualitatively altered embryo, and therefore, in the process of evolution, intrauterine development has been developed in mammals, and in most animals of this subtype, a secondary loss of yolk by eggs is observed.

The most primitive mammals are oviparous (platypus, echidna). They have telolecithal eggs, meroblastic cleavage, so their embryogenesis is similar to the development of birds. At marsupials the eggs contain a small amount of yolk, but the embryo is born underdeveloped and its further development takes place in the maternal pouch, where the connection between the nipple of the mammary gland of the mother and the esophagus of the baby is established. Higher mammals are characterized by intrauterine development and nutrition of the embryo at the expense of the maternal organism, which was reflected in embryogenesis. The oocytes have almost completely lost their yolk for the second time; they are considered secondarily oligolecithal, isolecithal. They develop in the follicles (folliculus - sac, vesicle) of the ovary. After ovulation (the rupture of the wall of the follicle and the release of the egg from the ovary), they enter the oviduct.

Oocytes in mammals are microscopic in size. Their diameter is 100 - 200 microns. They are covered with two shells - primary and secondary. The first is the plasmolemma of the cell. The second shell is the follicular cells. Of these, the wall of the follicle is built, where the eggs are located in the ovary. Fertilization of the egg takes place in the upper part of the oviduct. In this case, the shells of the egg are destroyed under the influence of the enzymes of the sperm acrosome. After internal fertilization, the formation of the first two blastomeres usually takes more time, because. more difficult process differentiation in the zygote (in humans up to 28 hours). As a result of differentiation, the material moves inside the zygote, fields are formed, from which certain rudiments will be formed in the future.

After the formation of the first cleavage furrow, two blastomeres are formed, which differ in size and contrast (one dark, the other light). One of the blastomeres contains the material of the trophoblast, the future provisional organ, and it is more homogeneous, while the other blastomere contains the material of the future embryoblast, so it is more complex in composition. Light blastomeres are crushed faster than dark ones and begin to overgrow them. Therefore, during subsequent crushing, not 4 blastomeres are formed, but 3, then 5, 1. i.e. Blastomeres are divided unevenly, and this type of crushing is called asynchronous. As a result of crushing, an embryo is formed in the form of a dense nodule - sterroblastula (at this moment it does not yet have a cavity).

Cleavage in higher mammals is complete, asynchronous: an embryo is formed, consisting of 3, 5, 7, etc. blastomeres. The latter usually lie in the form of a bunch of cells. This stage is called morula. Two types of cells are distinguishable in it: small - light and large - dark. Light cells have the highest mitotic activity. Intensively dividing, they are located on the surface of the morula in the form of the outer layer of the trophoblast (trophe - nutrition, blastos - sprout). Dark blastomeres divide more slowly, so they are larger than light ones and are located inside the embryo. The outer cells are lighter, and they form the trophoblast. The inner cells are darker and form an embryoblast.

Because the embryo does not have nutritional material, then the cells of the trophoblast, moving along the genital tract, begin to secrete enzymes and break down the mucus of the genital tract and absorb it. As a result, products of this cleavage appear inside the embryo, which gradually move the embryoblast material away - a small cavity appears and from that time on the embryo takes the form of a bubble - a blastocyst. It is in suspension, and the cavity increases, and the cells of the embryoblast seem to float above the cavity at its upper pole.

Only after this stage, in higher mammals, changes begin to occur in the internal cells of the embryo, i.e. in the embryoblast. Its cells split into 2 plates (gastrulation by delamination), the inner plate is the endoderm, and the outer one is the ectoderm and mesoderm. The trophoblast above the embryo is resorbed and this area occupies the outer germinal layer.

Late gastrulation

Gastrulation proceeds in the same way as in reptiles, birds, and lower mammals. The ectoderm and endoderm are formed by delamination of the germinal disc. If these sheets were formed from the material of the germinal shield, then they are called germinal, and if they originated from the non-embryonic zone of the germinal disc, then they are not germinal. Non-embryonic ectoderm and endoderm grow along the inner surface of the trophoblast. Soon the trophoblast, located above the embryo, resolves and the latter turns out to be lying in the uterine cavity for some time uncovered. In the anterior part of the germinal shield, blastomeres are intensively formed, which begin to move towards the posterior part, forming the primary streak, the primary nodule, the presumptive material of the notochord and the neural plate. This is followed by the formation of the mesoderm, notochord and neural tube.

The formation of the mesoderm proceeds in the same way as in birds. The cells of the marginal zone of the discoblastula migrate in two streams to the rear of the embryo. Here these streams meet and change their direction of movement. Now they move forward in the center of the germinal disc, thus forming a primary strip with a longitudinal depression - the primary groove. At the anterior end of the primary strip, a Hensen's nodule is formed with a depression - the primary fossa. In this zone, the material of the future chord turns up and grows forward between the ectoderm and endoderm in the form of a head (chordal) process.

The mesoderm develops from the cells of the primary streak. After migration, its material grows between the ectoderm and endoderm and turns into segmented mesoderm (somites), segmented legs adjacent to it, and non-segmented mesoderm. Somites consist of sclerotome (ventromedial part), dermotome (lateral part), myotome (medial part). Somites can connect to non-segmented mesoderm via segmented pedicels. The non-segmented part of the mesoderm looks like a hollow bag. Its outer wall is called the parietal leaf, and the inner one is called the visceral. The cavity enclosed between them is called the secondary cavity of the body, or coelom.

Then the trunk fold is formed; the amniotic fold is formed with the formation of the amnion and the creation of an aquatic environment for the development of the embryo. A yolk sac is formed that does not contain yolk, therefore, instead of a trophic function, it performs hematopoietic and reproductive functions. From the caudal section of the intestinal tube, allantois is also formed, which has lost its excretory function.

Trophoblast forms villi. The parietal mesoderm grows to it, which is introduced into the villi of the trophoblast and blood vessels are formed in it. From this moment on, the trophoblast turns into a chorion, the villi of which are introduced into the mucous membrane of the uterus and together with it form the placenta - a new provisional organ.

Features of the development of mammals is the early development of the trophoblast, and its further transformation into the chorion. Also new is the formation of the placenta (an analogue in birds is the serous membrane). Thus, in all mammals, gastrulation is divided into two stages. The first is almost hidden, but as a result of it, extra-embryonic material is evicted, which goes to build extra-embryonic organs. The second stage is actually gastrulation.

Formation of extra-embryonic (temporary) organs

One of the features of the development of mammals is that with isolecithal egg and holoblastic fragmentation, the formation of temporary organs occurs. As is known, provisional organs in the evolution of chordates are the acquisition of vertebrates with telolecithal, polylecital eggs and meroblastic cleavage.
Another feature of the development of mammals is the very early separation of the germinal from the non-embryonic part. So, already at the beginning of crushing, blastomeres are formed that form an extraembryonic auxiliary membrane - the trophoblast, with the help of which the embryo begins to receive nutrients from the uterine cavity. After the formation of germ layers, the trophoblast located above the embryo is reduced. The unreduced part of the trophoblast, growing together with the ectoderm, forms a single layer. Adjacent to this layer from the inside, sheets of non-segmented mesoderm and extraembryonic ectoderm grow.

Simultaneously with the formation of the body of the embryo, the development of the fetal membranes occurs: the yolk sac, amnion, chorion, allantois. The yolk sac, like in birds, is formed from the extraembryonic endoderm and the visceral mesoderm. Unlike birds, it does not contain yolk, but a protein liquid. Blood vessels form in the wall of the yolk sac. This shell performs the functions of hematopoiesis and trophic function. The latter is reduced to the processing and delivery of nutrients from the mother's body to the embryo. The duration of the function of the yolk sac varies from animal to animal.

As in birds, in mammals, the development of fetal membranes begins with the formation of two folds - the trunk and amniotic. The trunk fold lifts the embryo above the yolk sac and separates its embryonic part from the non-embryonic part, and the embryonic endoderm closes into the intestinal tube. However, the intestinal tube remains connected to the yolk sac by a narrow yolk stalk (duct). The point of the trunk fold is directed under the body of the embryo, while all the germ layers are bent: ectoderm, non-segmented mesoderm, endoderm.

The trophoblast, fused with the extraembryonic ectoderm and the parietal mesoderm, participates in the formation of the amniotic fold. The amniotic fold has two parts: inner and outer. Each of them is built from the sheets of the same name, but differs in the order of their arrangement. So, the inner layer of the inner part of the amniotic fold is the ectoderm, which in the outer part of the amniotic fold will be outside. This also applies to the sequence of occurrence of the parietal sheet of the mesoderm. The amniotic fold is directed over the body of the embryo. After the fusion of its edges, the embryo becomes immediately surrounded by two fetal membranes - the amnion and the chorion.

Types of placenta

During the development of the embryo in a mammal, certain contacts of the fetus with maternal tissues occur, i.e. the "mother-fetus" system is formed and this connection is carried out through the provisional organ - the placenta.

The placenta has undergone changes in the course of evolution. In birds, this was a serous membrane. In lower mammals, this is the trophoblast, which, improving, turns into the chorion and then into the placenta. Contact with maternal tissues of the chorion is different, therefore, there are four main types of placenta.

1. In the lower (in pigs), the chorionic villi are in contact with the entire surface of the uterine mucosa and directly with its epithelium, and this type of placenta is called epitheliochoriatic. In this case, the epithelium of the uterine mucosa is not destroyed. Anatomically, such a placenta is called diffuse, because. the entire mucosa is involved and the villi are arranged one at a time.

2. Ruminants have a desmochorial type of placenta. Here, the chorionic villi come into contact with the connective tissue, growing into the epithelium, which is destroyed. The connection is more complex, strong and close. The villi are distributed in the form of cotyledons (clusters), and not diffusely, so such an anatomically called cotyledon (multiple) placenta.

3. The third type of placenta is endotheliochorial. Found in predators. Chorionic villi grow to the endothelium of blood capillaries, partially destroying the walls of blood vessels. Contact with the mother's body is even closer, the villi are already concentrated in the form of a belt, occupying part of the endometrium. This type of placenta is anatomically called cingulate.

4. In primates, rodents, there is a hemochorial type of placenta. The chorionic villi are in contact with maternal blood. During the formation of the placenta, the epithelium is destroyed, then grows into connective tissue and destroys it, blood vessels are also destroyed. The blood leaves the blood vessels, thus forming lacunae (lakes) with which the villi are in contact. This is a more perfect form of the placenta. The villi are already located on a small area, forming a placenta in the form of a disk or cake (in humans, 2-3 cm thick). Anatomically, this type of placenta is called discoid.

The placenta performs the following functions:

Trophic;

Respiratory; fertilization mammals gastrulation placenta

excretory;

Immunobiological - protection of the fetus from antigens that may be in the mother's blood. But this protection is poor, so suppressor cells work intensively in the mother's body. suppressing maternal immunity, so pregnancy takes place against the background of immunodeficiency (from the day of fertilization);

Barrier - the placental barrier is unstable for many compounds and a number of drugs, as well as for alcohol;

Endocrine - the placenta begins to produce hormones early that support the process of embryonic development;

Protein-synthesizing function, according to this function, all placentas can be divided into two types:

Type 1 - epitheliochorial and desmochorial. With these types of placenta from the mother's body, complex compounds are absorbed from the blood. Then, in the placenta, they are broken down to simple ones and in this form they enter the fetus, where embryo-specific or “organ-specific compounds are synthesized, that is, the organs build themselves. Therefore, by the time of birth, the organs of the fetus are more mature.

Type 2 - endotheliochorial and hemochorial. Simple compounds are taken from the mother's blood, so during pregnancy there is no particular danger to the mother's body. For example, with histosis, there are no deaths. In the placenta, complex compounds are synthesized from these simple compounds, and in finished form it is delivered to the fetus (after the 7th month of embryogenesis, the fetus itself synthesizes some of the compounds it needs). Therefore, by the time of birth, such an organism is functionally less mature.

The period of embryonic development is most complex in higher animals and consists of several stages.

The period starts with crushing of the zygote(Fig. 1), i.e., a series of successive mitotic divisions of a fertilized egg. The two cells formed as a result of division (and all their subsequent generations) at this stage are called blastomeres. One division follows another, and there is no growth of the resulting blastomeres, and with each division the cells become smaller and smaller. This feature of cell divisions determined the appearance of the figurative term "zygote splitting".

Rice. 1.Cleavage and gastrulation of the lancelet egg (side view)

The figure shows: A- a mature egg with a polar body; b- 2-cell stage; V- 4-cell stage; G- 8-cell stage; d- 16-cell stage; e- 32-cell stage (in section to show the blastocoel); g - blastula; h - section of the blastula; and - early gastrula (at the vegetative pole - arrow - invagination begins); j - late gastrula (invagination ended and blastopore formed; 1 - polar body; 2 - blastocoel; 3 - ectoderm; 4 - endoderm; 5 - cavity of the primary intestine; 6 - blastopore).

As a result of crushing (when the number of blastomeres reaches a significant number), a blastula is formed (see Fig. 1, g, h). Often it is a hollow ball (for example, in a lancelet), the wall of which is formed by one layer of cells - the blastoderm. The cavity of the blastula is the blastocoel, or primary cavity, filled with fluid.

At the next stage, the process of gastrulation is carried out - the formation of the gastrula. In many animals, it is formed by invagination of the blastoderm inward at one of the blastula poles during intensive multiplication of cells in this zone. As a result, a gastrula appears (see Fig. 1, i, j).

The outer layer of cells is called the ectoderm, and the inner layer is called the endoderm. The internal cavity bounded by the endoderm, the cavity of the primary intestine communicates with external environment primary mouth, or blastopore. There are other types of gastrulation, but in all animals (except sponges and coelenterates), this process ends with the formation of another cell layer - the mesoderm. It is laid between the ento- and ectoderm.

At the end of the gastrulation stage, three cell layers appear (ecto-, endo- and mesoderm), or three germ layers.

Then the processes of histogenesis (formation of tissues) and organogenesis (formation of organs) begin in the embryo (embryo). As a result of cell differentiation of the germ layers, various tissues and organs of the developing organism are formed. From the ectoderm, integuments and the nervous system are formed. Due to the endoderm, the intestinal tube, liver, pancreas, and lungs are formed. The mesoderm produces all other systems: musculoskeletal, circulatory, excretory, sexual. The discovery of homology (similarity) of three germ layers in almost all animals served as an important argument in favor of the point of view about the unity of their origin. The patterns outlined above were established at the end of the 19th century. I. I. Mechnikov and A. O. Kovalevsky and formed the basis of the “doctrine of germ layers” formulated by them.

During the embryonic period, there is an acceleration in the rate of growth and differentiation in the developing embryo. Only in the process of crushing the zygote, growth does not occur, and the blastula (in its mass) can even be significantly inferior to the zygote, but starting from the process of gastrulation, the mass of the embryo rapidly increases.

The formation of heterogeneous cells begins at the stage of crushing and underlies the primary tissue differentiation - the emergence of three germ layers. Further development of the embryo is accompanied by an increasingly intensifying process of differentiation and morphogenesis. By the end of the embryonic period, the embryo already has all the main organs and systems that ensure viability in the external environment.

The embryonic period ends with the birth of a new individual capable of independent existence.

The development of the mammalian embryo goes through stages characteristic of vertebrate amniotes. Lancelet, amphibians, fish are anamniotes. They do not have an amnion. They do not need it, since their development takes place in a natural aquatic environment. Early embryogenesis occurs in the oviducts, and final development occurs in the uterus. The uterine period of development is divided into two periods: embryonic and fetal. The duration of the uterine period in different classes of mammals is different, from 2-3 months to a year. In mammals, in parallel with the development of the embryo, the formation of extra-embryonic organs takes place, which ensure the development of the embryo.

During the pre-embryonic period, germ cells are formed gametogenesis (progenesis)). The formation and growth of female germ cells takes place in the ovary, from where they are ejected into the abdominal space at the stage of the 1st order oocyte and are captured by the villi (fimbriae) of the fallopian tubes. The first division of maturation begins at the time of ovulation, and meiosis ends in the lumen of the fallopian tube (oviduct).

As a result of the first division of maturation (reduction), the 1st order oocyte turns into the 2nd order oocyte, which has a haploid set of chromosomes. As a result of the second division of maturation, an oocyte of the 2nd order turns into a mature female germ cell - an oocyte, which remains viable from several hours to 1 day.

In most cases, one germ cell matures in each of the ovaries. With the simultaneous maturation of two or more germ cells in some classes, the formation of several embryos is possible - a multiple pregnancy. The mammalian egg is secondarily isolecithal, has a rounded shape, is surrounded by a shiny membrane and a layer of follicular cells forming a radiant crown. The cytoplasm of the egg is fine-grained and contains a small amount of yolk grains. The diameter of the egg is on average 120-150 microns.

Male germ cells (flagellated spermatozoon) develop in the convoluted tubules of the testes (testes or testicles), enter the vas deferens, and have a haploid set of chromosomes. At the same time, millions of them develop, then they enter the vas deferens, where they are deposited. The spermatozoon consists of a head, neck, body, tail in the form of a flagellum and in their organization differ little in different types placental animals: head shape, size.

The development of the early stages of embryogenesis (fertilization, crushing and the first stage of blastulation) occurs in the oviducts (fallopian tubes).

Fertilization: monospermia, not free - in the ampullar part of the oviducts.

Splitting up: complete, uneven, wrong. As a result, after the first division, two types of blastomeres are formed. Small light ones are embryoblasts, and large dark ones are trophoblasts.

Blastulation proceeds in two stages. 1) the formation of a dense blastula or blastocyst in the form of a berry (morula). The appearance of the blastula is rounded. Embryoblast cells are located in the center. An embryo will develop from them. Along the periphery are located in one layer of trophoblast cells with microvilli. They actively absorb nutrients from the tissue fluid of the oviducts, providing nutrition to the embryo. At this stage, the embryo from the oviducts enters the uterine cavity. The glands of its mucous membrane produce a mucous secret - royal jelly containing nutrients. Trophoblast cells actively absorb its components and transfer them to embryoblast cells. The embryo floats in the uterine cavity. Excess trophic material accumulates and compresses the embryoblast in the form of a disk. This second stage of blastulation is called the blastocyst.

Subsequently, the processes of development of the embryo proceed in parallel, i.e. contemporaneously with the development of the germinal membranes.

gastrulation in mammals occurs in two stages, as in birds.

Stage 1 - delamination: splitting of the germinal disk into two sheets or bookmarks: ectoderm and endoderm. At the same time, the ectoderm moves to the trophoblast and displaces it above itself, i.e. incorporated into the trophoblast. The cells of the trophoblast above it are exfoliated - Rauber's leaf. In the middle part of the two-leafed embryo, the germinal shield stands out. Actively dividing cells, especially at the anterior margin of the germinal shield. Cells move along the sides of the embryo to the rear edge, two streams collide, forming a primary streak. Its cells divide by mitosis, invaginate towards the endoderm. In this area, two leaves grow together. The cells between the leaves, continuing to divide, grow with wings between the ectoderm and endoderm, forming a mesodermal anlage. On the surface of the cells of the primary streak, they divide by mitosis and rush to the anterior edge of the embryo. But since the density of cellular material at the anterior margin is high, the cells of the primitive streak accumulate to form a Hensen's node. Its cells, continuing to divide by mitosis, migrate to the endoderm and grow forward between the wings of the mesoderm. Thus, at the gastrula stage, the first axial organ, the chord, is immediately formed. The remnants of the Hensen's knot cells degenerate on the surface of the ectoderm to the anterior margin, forming a neural anlage. Thus, at the stage of gastrulation, embryonic anlages were formed - the sources of tissue development.

Formation of axial organs going on general principle like the lancelet. At this stage, the process of histogenesis begins - the development of tissues. In the area of ​​axial organs from the material of the tabs from which they are formed.

Formation of the body of the embryo and embryonic membranes(provisional organs occur, as in birds, with the help of trunk and amniotic folds. Due to two lateral and two anterior-posterior trunk folds, body(torso) and yolk sac. It does not contain yolk. Trunk folds are formed in the area of ​​fusion of the trophoblast and ectoderm. At the same time, cells in the area of ​​contact between the trophoblast and ectoderm begin to move in the opposite direction from the trunk folds to the dorsal surface of the embryo, forming amniotic folds, there are also four of them. Thus, the ectoderm remains inside, but is divided into the germinal ectoderm and the ectoderm that forms the amnion wall. The crests of the amniotic folds fuse together. As a result of their fusion around the embryo, a cavity is formed in the form of a bowl - amnion. Gradually, it is filled with liquid, in which the further development of the embryo will take place. The amnion grows in the extraembryonic cavity of the coelom, reaching the greatest development in comparison with other membranes. From the outer surface, after the fusion of the amniotic folds, a chorion(similar to the serous membrane). The surface of the chorion is divided into two parts: smooth and villous. Smooth chorion performs a protective function. The villous chorion faces the uterine mucosa. And soon it establishes contacts with the uterine mucosa that are specific to different classes of mammals. The chorionic villi form the fetal part of the placenta. The second part is maternal. The structures of the uterine mucosa, with which the chorionic villi come into contact, will be different for different classes, so there are four types of placentas. At the same time, due to the protrusion of the posterior wall of the intestine into the extraembryonic cavity, the coelom is formed allantois. In mammals, it does not reach great development. Growing, the amnion compresses the yolk sac and allantois in the form of a funiculus. The walls of the yolk sac and allantois grow together. This is how the umbilical cord is formed. In their common wall, umbilical vessels are formed: two arteries and one vein. In mammals, such as the pig, the lumens of the allantois and the yolk sac do not fuse. In sections of the umbilical cord, they are visible. The yolk sac is lined with squamous epithelium, while the allantois is lined with cuboidal epithelium. The walls of blood vessels have their own membranes. The umbilical cord fuses with the chorionic villi. Its vessels grow into the stroma of the villi. The blood of the fetus and mother does not mix.